A considerable amount of research has been directed toward the development of organic polymeric materials which exhibit second-order nonlinear optical (NLO) and linear electro-optical (EO) properties. Such materials are useful in the production of efficient, ultrafast, and low voltage electro-optical devices, such as modulators, switches, and tunable filters, to name a few. Among the advantages of incorporating organic polymers into electro-optical devices over traditional materials, such as inorganic-based crystals, include ease of processing and device fabrication with low production costs. To be useful in electro-optical devices, organic polymeric materials must exhibit high NLO coefficients, demonstrate good long-term stability of the NLO or EO effect, have good structural and chemical stability, and show low optical propagation losses.
In general, organic polymers exhibiting nonlinear optical and electro-optical properties are produced by incorporating into the polymer a dye chromophore having nonvanishing microscopic hyperpolarizability and macroscopic polar order. Such dyes must possess strong optical nonlinearities along a single charge transfer axis, a requirement which can be met by including a donor group at one end of a .pi.-conjugated electron system and an acceptor group at the other end.
Incorporation of these nonlinear optical chromophores (dyes) into polymers can be accomplished by dissolving the chromophore into a polymer host (guest/host systems), by covalent bonding of the chromophore as a sidechain onto the polymer backbone or main chain, or by using the chromophore to cross-link polymer chains. Because guest/host systems are the simplest NLO polymer systems to create, many guest/host systems have been investigated, with poly(methyl methacrylate) or polystyrene as the most prominent hosts together with typical donor-acceptor substituted chromophores, such as N-ethyl-N-hydroxyethylamino-nitroazobenzene. See, for example, D. Burland et al., Chem. Rev. 94, 31-75 (1994); see also S. R. Marder & J. W. Perry, Science 263 1706-1707 (1994). In addition, second-order nonlinear optical properties of dye-attached polymers were disclosed by Amano et al. in U.S. Pat. No. 5,359,008 and were reported by P. Kaatz et al., Macromolecules 29, 1666-1678 (1996). Examples of such dye-appended polymers include azo-dye-attached poly(methyl methacrylate) and polyimides, stilbene-dye attached poly(methyl methacrylate), and dye-appended polysiloxanes, polyacrylates, polyesters, polyurethanes, polyamides, polystyrenes, polycarbonates, polyethers, and the like.
To enhance second-order nonlinear optical properties, dye-containing polymers are typically "poled". In the technique of "poling", an external electric field is applied to break the isotropic symmetry of the polymers during which the NLO dye chromophores are aligned by coupling to their dipole moment. This poling procedure imposes noncentrosymmetry on the polymer material. The desired noncentrosymmetry is most easily induced at temperatures close to the glass transition temperature, T.sub.g, of the polymer because of the increased mobility of the NLO dye molecules in the softening polymer matrix. Cooling is then performed in the presence of the applied electric field, which results in the formation of a polymer glass at the lower temperatures. A temperature-stable and oriented system is thereby provided.
To meet device stability requirements, polymers having a very high glass transition temperature (T.sub.g &gt;150.degree. C.) are typically chosen, and very large dyes having a high melting point are typically appended to or mixed with the polymer. It is important for use in second-order nonlinear optical applications that the poled polymers remain in their poled configuration over time, and in general, a polymer having a high T.sub.g provides a stronger frame to prevent relaxation of the aligned dyes which were poled by the electrical field. Thus, a nonlinear optical/electro-optical polymer with a higher T.sub.g usually has a slower relaxation time at a given temperature. However, in many known polymers, such as poly(methyl methacrylate)-based compositions, the alignment is thermodynamically unstable and decays quickly, resulting in greatly reduced nonlinearity.
In addition to thermal stability requirements, electro-optical devices require polymeric compositions which have high NLO and EO coefficients, thereby necessitating the use of dyes exhibiting high NLO and EO activity. However, the trade-off is that dyes exhibiting high activity normally have lower thermal stability, thereby limiting the poling temperature. Additionally, at high temperatures, device processes such as generating channel waveguides become more difficult than at lower temperatures.
Thus, a need exists for NLO and EO organic polymer compositions which exhibit not only high T.sub.g 's but also improved long-term stability at elevated temperatures. Such polymeric materials should also exhibit large electro-optical and nonlinear optical coefficients, comparable to those of inorganic crystals such as lithium niobate, but without the associated drawbacks, i.e. difficulty in growing, vulnerability to cracking, and high expense. Organic polymers fulfilling these requirements would be useful in the development of optoelectronics devices having higher data rates, all optical as well as electro-optical switching, and high parallel capacity logic functions. In addition, such polymers would be useful in the development of fiber optic communications systems and optical computation and parallel optical image processing systems.
The novel chiral polymer compositions of the present invention, which include nonlinear optical chromophores incorporated therein, meet the aforementioned needs.